Conventionally, a motor driving device of this type drives a motor under pulse width modulation (PWM) control as follows. If a driving speed of the motor is higher than a target speed, the device reduces an ON time of PWM, whereas if the driving speed of the motor is lower than the target speed, the device increases the ON time.
Moreover, in a refrigerator that performs a cooling operation by using a conventional motor driving device, a four-way valve is provided in a refrigeration cycle. When the compressor is operated, the four-way valve is operated by a normal refrigeration cycle. When the compressor is stopped, the four-way valve is switched such that a high pressure side and low pressure side are separated from each other on the cycle, that a high-pressure refrigerant is supplied from a dryer to the compressor, and that a pressure difference between suction and discharge of the compressor becomes small. This configuration prevents the refrigerant on the high-pressure side from flowing into an evaporator at the time of stoppage of the compressor and keeps a temperature of the evaporator low to prevent a rise in refrigerator temperature, thereby achieving energy saving in the refrigerator (see, for example, PTL 1).
Moreover, in general, the motor is driven as follows at the time of starting the motor. That is, the motor is started while sequentially switching predetermined patterns of an applied voltage to the motor in a predetermined cycle. Then, when a rotation speed of the motor reaches a set rotation speed, the patterns of the applied voltage to the motor are switched for control that is based on position detection such as detection of a magnetic pole position of the motor, and then the motor is driven (see, for example, PTL 2).
FIG. 17 shows an internal configuration of a refrigerator using the conventional motor driving device described in PTL 1.
As shown in FIG. 17, in the conventional refrigerator, a refrigeration cycle is formed of low-pressure shell compressor 101, condenser 102, dryer 103, capillary tube 104 and evaporator 105 in this order. The refrigerant flows in the refrigeration cycle from compressor 101 to condenser 105.
Four-way valve 106 connects inlet A and the dryer 103 to each other, connects outlet B and capillary tube 104 to each other, connects inlet C and evaporator 105 to each other, and connects outlet D and compressor 101 to each other. During operation of compressor 101, four-way valve 106 causes inlet A to communicate with outlet B, and causes inlet C to communicate with outlet D. Moreover, during stoppage of compressor 101, four-way valve 106 causes inlet A to communicate with outlet D, and causes inlet C to communicate with outlet B. This forms: a closed circuit in a high-pressure area provided with compressor 101, condenser 102 and dryer 103; and a closed circuit in a low-pressure area provided with capillary tube 104 and evaporator 105 during the stoppage of compressor 101.
During a refrigeration cycle operation, a normal refrigeration cycle is formed to enable a regular cooling operation. Moreover, at the time of stoppage of the refrigeration cycle, compressor 101 can be started in a state in which the high pressure side and the low pressure side are separated from each other on the cycle, the high-pressure refrigerant is supplied from the dryer to the compressor 101, the pressure difference between suction and discharge of compressor 101 is reduced, and load torque fluctuations are small. This configuration prevents the high-pressure side refrigerant from flowing into evaporator 105 and the rise in temperature of evaporator 105 during the stoppage of the refrigeration cycle. This makes it possible to reduce a loss of energy in the refrigeration cycle.
However, in such a configuration of the refrigerator using the conventional motor driving device as shown in PTL 1 and PTL 2, the motor driving device cannot cope with large load torque fluctuations at the time of starting, vibrations increase, and the reliability of compressor 101 decreases. Therefore, in order to stably start compressor 101, it is necessary to balance a suction pressure and discharge pressure of compressor 101 by using four-way valve 106 when compressor 101 is stopped. As a result, there is a problem that a system becomes complicated and cost also increases.
Moreover, the conventional motor driving device and the refrigerator using the same are configured to detect a rotational position of a rotor of the brushless DC motor and to switch a stator winding to be energized based on the rotational position thereof. In drive of the brushless DC motor in a special environment such as a compressor, for the detection of the rotational position of the rotor, a detector such as an encoder and a Hall element is not used, but in general, there is used a digital sensorless mode for comparing an inverter output voltage and ½ of an inverter input voltage with each other and detecting a point where a magnitude relationship therebetween changes (for example, see NPL 1).
FIG. 18 shows a block diagram of a motor driving device of NPL 1.
In FIG. 18, the conventional motor driving device uses commercial power source 181 as an input, converts an alternating current (AC) voltage into a direct current (DC) voltage by a rectifying/smoothing circuit 182, and inputs the DC voltage to inverter 183. In inverter 183, six switching elements 183a to 183f are connected to one another in the form of a three-phase full bridge, and diodes 183g to 1831 are respectively connected in parallel in an opposite direction to switching elements 183a to 183f. In this way, inverter 183 converts the DC input into three-phase AC power and supplies the power to brushless DC motor 184. Position detection circuit 185 detects a relative position of the rotor based on terminal voltages of brushless DC motor 184.
FIG. 19 is a circuit diagram of position detection circuit 185 of the motor driving device of NPL 1.
In FIG. 19, position detection circuit 185 in NPL 1 is composed of comparator 186 realized by comparators. The terminal voltages of brushless DC motor 184 are input to non-inverting inputs of comparator 186, and a voltage of ½ of the inverter input voltage is input as a reference voltage to inverting inputs of comparator 186. For a position signal, with regard to an induced voltage appearing at such an inverter output terminal in a non-energized phase in the stator winding, timing at which a magnitude relationship of the induced voltage with the reference voltage changes (that is, a zero-cross point of the induced voltage) is detected, and a detection result is output.
FIG. 20 is a diagram showing waveforms including current waveform A and terminal voltage waveform B at a time of sensorless driving of the motor driving device according to NPL 1. It is graph C that shows a comparison result showing a magnitude relationship of terminal voltage waveform B with the reference voltage (½ voltage of the inverter input). Output waveform D of position detection circuit 185 is a waveform obtained by removing an influence of switching by PWM control and an influence of spike voltage X and spike voltage Y from such a waveform C by waveform processing. Here, spike voltage X and spike voltage Y are generated when energy of the winding to which voltage supply is interrupted by commutation is released as a reflux current. Timing (a rising edge or a falling edge) at which a signal state of waveform D changes is detected as position detection, and brushless DC motor 184 can be stably driven by repeating the commutation based on this position signal.
FIG. 21 is a block diagram showing a conventional motor driving device described in PTL 3.
As shown in FIG. 21, the conventional motor driving device includes: brushless DC motor 214 composed of a rotor having a permanent magnet and a stator having a three-phase winding; inverter 213 that supplies power to the three-phase winding; and driver 215 that drives inverter 213. The conventional motor driving device further includes a position detector 216 that detects a relative rotational position of the rotor based on an induced voltage generated in a stator winding of brushless DC motor 214 and then outputs a position signal. Furthermore, the conventional motor driving device includes: first waveform generator 217 that outputs a rectangular wave or a sine wave or a waveform similar to these waveforms while performing duty control based on the signal output from position detector 216; and a second waveform generator 219 that outputs a rectangular wave or a sine wave or a waveform similar to these waveforms to brushless DC motor 214. Moreover, the conventional motor driving device includes switching determiner 219 that drives inverter 213 by the output of first waveform generator 217 when brushless DC motor 214 is rotating at a low speed equal to or lower than a predetermined rotation speed, and drives inverter 213 by the output of second waveform generator 218 when brushless DC motor 214 is rotating at a high speed exceeding the predetermined rotation speed. Furthermore, the conventional motor driving device is configured to output a pattern for detecting the induced voltage of brushless DC motor 214 at predetermined timing when driven by second waveform generator 219.
With such a configuration, in the conventional motor driving device, at the low speed, brushless DC motor is subjected to high-efficiency driving of performing sensorless driving by first waveform generator 217 based on the signal of position detector 216, and at the high speed, brushless DC motor 214 is subjected to frequency-fixed synchronous driving by second waveform generator 218. Moreover, in the conventional motor driving device, position detector 216 periodically obtains position information of the rotor based on the detection of the zero-cross point of the induced voltage in brushless DC motor 214 and determines commutation timing. Therefore, stable driving performance is obtained even at a time of high load and high speed driving by the synchronous driving.
However, in the configuration of the conventional motor driving device shown in each of NPL 1 and PTL 3, the current flowing through the motor winding is large under conditions that low-speed high torque is required at such a starting time in the case of the sensorless driving. When the motor winding is switched by the commutation, it takes time until the energy of the winding to which the supply of power is interrupted is consumed as a reflux current.
In FIG. 20, for example, timing of commutation from section K2 to section K3 will be considered. At a time of shift from section K2 to section K3, when energization to a U-phase winding supplied with power is cut off, energy accumulated in the U-phase winding is refluxed through an inside of brushless DC motor 184 via the switching element 183f and the diode 183j, which are shown in FIG. 18, and is then consumed. Hence, diode 183j turns to a conductive state, and is thereby connected to a negative side of the inverter input voltage, and accordingly, spike voltage Y shown in FIG. 20 is generated in the terminal voltage waveform at the time of generation of the reflux current.
In a similar way, at a time of shift from section K4 to section K5, winding energy is consumed as the reflux current via switching element 183c and diode 183g, diode 183g is connected to a positive side of the inverter input voltage, and spike voltage X shown in FIG. 20 is generated.
FIG. 22 shows current waveform A0 and terminal voltage waveform B0, which are waveforms when the conventional motor driving device drives brushless DC motor 214 in a state in which the motor current in the sensorless driving is large. In the conventional motor driving device as shown in FIG. 22, since the current flowing through brushless DC motor 214 is high, the energy accumulated in the U-phase winding as a result that the supply of power to the U-phase winding itself is interrupted is large. Therefore, a release time of the energy, that is, each of a generation period of spike voltage X0 and spike voltage Y0, which are shown in FIG. 22, becomes longer.
Hence, as shown in terminal voltage waveform B0 of FIG. 22, spike voltage X0 and spike voltage Y0 cover and hide the zero-cross point of the induced voltage, so that the position signal cannot be detected.
As a result, in the driving in such a state in which the motor current is large in the sensorless driving, the motor driving device as shown in NPL 1 cannot perform accurate position detection for brushless DC motor 184 Therefore, there are problems such as a decrease in drive torque, a decrease in starting performance due to the decrease in torque, a decrease in motor drive efficiency, a decrease in speed stability, and increases in vibration and noise due to speed fluctuations.
Moreover, in such a configuration of the motor driving device as described in PTL 3, a signal of a special pattern driven by an inverter is output during synchronous driving. In this way, it is made possible to acquire the position signal of brushless DC motor 214, and driving stability at high speed and high load is ensured. However, this configuration cannot serve for improving stability in such a driving state in which the motor current is large and the spike voltage covers and hides a zero crossing signal during the sensorless driving. Hence, in the conventional motor driving device as described in PTL 3, at the time of the sensorless driving when the motor current is high, there are problems similar to those in NPL 1 mentioned above.